The invention relates to a resilient tire capable of supporting a vehicle load by the structural components of the tire without the benefit of internal air pressure.
The pneumatic tire has been the solution of choice for vehicular mobility for over a century. Modem belted, radial carcass pneumatic tires are remarkable products that provide an effective means for supporting applied loads while allowing reasonable vertical and lateral compliance. The pneumatic tire obtains its mechanical attributes largely due to the action of internal air pressure in the tire cavity. Reaction to the inflation pressure provides correct rigidities to the belt and carcass components. Inflation pressure is then one of the most important design parameters for a pneumatic tire. Unfortunately, when inflation pressure is fixed, the designer of a pneumatic tire has limited flexibility to modify the vertical stiffness of the tire.
Good pressure maintenance is required to obtain the best performance from a pneumatic tire. Inflation pressure below that specified can result in a loss of fuel economy. Of primary importance is that a conventional pneumatic tire is capable of very limited use after a complete loss of inflation pressure. Many tire constructions have been proposed for continued mobility of a vehicle after a complete loss of air pressure from the tire. Commercially available runflat tire solutions are pneumatic tires having added sidewall reinforcements or fillers to permit the sidewalls to act in compression as load supporting members during deflated operation. This added reinforcement often results in the disadvantages of higher tire mass and reduced riding comfort. Other attempts to provide runflat capability utilize essentially annular reinforcing bands in the tire crown portion. In these solutions, the rigidity of the tread portion results partly from the inherent properties of the annular reinforcing band and partly from the reaction to inflation pressure. Still other solutions rely on secondary internal support structures attached to the wheel. These supports add mass to the mounted assembly and either increase mounting difficulty or may require the use of multiple piece rims. All of these approaches are hybrids of an otherwise pneumatic tire structure and suffer from design compromises that are optimal for neither the inflated nor deflated states. In addition, these runflat solutions require the use of some means to monitor tire inflation pressure and to inform the vehicle operator if the inflation pressure is outside the recommended limits.
A tire designed to operate without the benefit of inflation pressure eliminates many of the problems and compromises associated with a pneumatic tire. There is only one operating condition, non-inflated. Neither pressure maintenance nor pressure monitoring is required. Structurally supported resilient tires such as solid tires or other elastomeric structures to date have not provided the levels of performance expected from a conventional pneumatic tire. A structurally supported resilient tire solution that delivers pneumatic tire-like performance would be a welcome improvement.
A structurally supported resilient tire in accordance with the invention supports its load solely through the structural properties of its tread, sidewall and bead portions, and without support from internal air pressure. The tread portion of a structurally supported resilient tire, when viewed without the sidewall and bead portions, appears as a reinforced annular band. The reinforced annular band has rigidities to resist bending in both the tire meridian and equatorial planes. A meridian plane passes through the tire with the axis of rotation lying wholly in the meridian plane. The equatorial plane passes perpendicular to the tire axis of rotation and bisects the tire structure.
The contact of an annular band with a flat plane is analogous to a tire contacting a ground surface. The resultant reactions are analogous to the ground contact stresses of a loaded tire. For a stiff annular band comprised of a homogeneous material, the pressure distribution satisfying the equilibrium and bending moment requirements is made up of a pair of concentrated forces located at each end of the contact area, one end of which is shown in FIG. 2A. In this idealization, no shear deformation of the annular band occurs. However, if the annular band comprises a structure which prescribes shear deformation, the resulting pressure distribution is substantially uniform.
A structurally supported resilient tire in accordance with the invention includes a tread portion, sidewall portions extending radially from the tread portion toward a tire axis, and bead portions at radially inner ends of the sidewall portions to anchor the tire to a wheel. The tread, sidewalls, and beads define a hollow, annular space, similar to that in a pneumatic tire. According to the invention, an annular band is disposed radially inward of the tread portion, the annular band comprising an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of said elastomeric shear layer, and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer. Preferably, the membranes comprise superposed layers of essentially inextensible cord reinforcements embedded in an elastomeric coating layer. The membranes have a longitudinal tensile modulus of elasticity sufficiently greater than the shear modulus of elasticity of the elastomeric shear layer such that, under an externally applied load, the ground contacting tread portion deforms from essentially a circular shape to a flat shape while maintaining an essentially constant length of the membranes. Relative displacement of the membranes occurs by shear in the shear layer.
This effect is schematically represented in FIG. 2B. As shown in FIG. 2B, a beneficial result is a more uniform ground contact pressure throughout the length of the contact area compared to other tires not using an annular band having the deformation properties just described. The annular band does not rely on internal inflation pressure to have a transverse stiffness in a tire meridian plane and a longitudinal bending stiffness in the tire equatorial plane sufficiently high to act as a load-supporting member.
According to one aspect of the invention, a transverse radius of the annular band, that is, the radius of curvature in the tire meridian plane, is less than the transverse radius of the outer tread surface to resist longitudinal buckling of the annular band in the contact area.
The structure according to the invention advantageously allows the tire designer to adjust the vertical stiffness of the tire somewhat independently of the contact pressure. In conventional pneumatic tires, by contrast, the ground contact pressure and tire vertical stiffness are strongly coupled.
The tire sidewalls provide the necessary structure to react at the wheel the load supported by the annular band, thus supporting the mass of a vehicle. In a conventional pneumatic tire, load support is provided by differences in tensions of the tire sidewalls, with the minimum sidewall tension being at the center of the contact area and the maximum being at a meridian opposite the contact area. As shown in FIG. 3a, the structurally supported resilient tire of the present invention supports its load by tensioning the sidewall for those meridians outside the contact area. Optimal load support is obtained when the sidewalls have a high effective radial stiffness in tension and a low effective radial stiffness in compression. When these conditions are satisfied, the wheel can be said to hang from the upper portion of the tire. In addition, for optimal load support, the sidewalls have a rectilinear profile and radially oriented reinforcing elements.
The vertical stiffness of the tire of the invention, which is the resistance under load to deformation in the vertical direction, can be affected to a significant degree by the counterdeflection stiffness of the tire. Counterdeflection stiffness is a measure of the resistance of the tire to deformation of the portion not in ground contact. Counterdeflection of the tire allows some vertical displacement of the wheel axis, which effectively decreases the vertical stiffness of the tire. Adjusting the counterdeflection stiffness of the tire adjusts the vertical stiffness of the tire.
When the tire of the invention rotates at high angular velocity, centripetal forces develop in the annular band. These forces result in circumferential stress, which tends to cause the annular band to expand radially outward. Expansion of the annular band is resisted by the high effective radial stiffness of the sidewalls. Since no such centripetal forces develop in the ground contact area, the net result is a vertically upward force, which acts to support a portion of the imposed load, and increases the effective vertical stiffness of the tire. The centripetal forces, and hence, the effective vertical stiffness of the tire, increase as speed increases; thus, the tire deflection is reduced as speed increases. Reduced deflection reduces heat generation in the tire and improves high-speed performance.
The tensions developed in the sidewalls of the tire of the invention when loaded are significantly lower than the sidewall tensions of an inflated and loaded pneumatic tire. Referring to FIG. 1, the bead portions 160 may employ any of several bead structures which allow proper seating on the rim 10 without relying on inflation pressure and which maintain proper seating of the bead portions during use of the tire. An example of a bead construction meeting these requirements is shown in U.S. Pat. No. 5,785,781 to Drieux et al and is incorporated by reference herein.
According to one embodiment of the invention, a structurally supported resilient tire comprises a ground contacting tread portion, sidewall portions extending radially inward from the tread portion and anchored in bead portions adapted to remain secure to a wheel during rolling of the tire, and a reinforced annular band disposed radially inward of the tread portion, the band comprising an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of the elastomeric shear layer and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer, and in which the second membrane is undulated having an amplitude of undulation in the radial direction and a wavelength of undulation in the axial direction.
This undulated membrane resists compressive buckling of the annular band in the ground contact area without constraints on the transverse radii of the annular band and outer tread surface. If the tread grooves coincide with the minimum of the undulation, that is, the portion of the membrane concave toward the tread, then these grooves can be deeper than the grooves of conventional tires, thus improving hydroplaning resistance of the tire.
According to another embodiment, a structurally supported resilient tire comprises a ground contacting tread portion, sidewall portions extending radially inward from the tread portion and anchored in bead portions adapted to remain secure to a wheel during rolling of the tire, and a reinforced annular band disposed radially inward of the tread portion, the band comprising an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of the elastomeric shear layer and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer, wherein a ratio of the longitudinal stiffness of the band in the tire equatorial plane to an effective radial stiffness of the sidewall portion in tension is less than 100:1.
According to yet another embodiment of the invention, a structurally supported resilient tire comprises a ground contacting tread portion, sidewall portions extending radially inward from the tread portion and anchored in bead portions adapted to remain secure to a wheel during rolling of the tire, and a reinforced annular band disposed radially inward of the tread portion, the band comprising an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of the elastomeric shear layer and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer, wherein the sidewall portions are essentially inextensible in tension and essentially without resistance to compressive buckling, whereby an externally applied load is supported substantially by tensile forces in the sidewall portion in the region of the tire out of contact with the ground and substantially without vertical load support from the sidewall portion in the region in contact with the ground.
According to the invention, a method for making a structurally supported resilient tire with a reinforced annular band having an elastomeric shear layer between longitudinally stiff membranes, comprises the steps of selecting a ground contact pressure and tire radius, multiplying the ground contact pressure by the tire radius to determine a shear layer factor, selecting a shear layer material having a shear modulus of elasticity and with a thickness so that the product of the shear modulus of elasticity times the thickness is equal to the shear layer factor, selecting membranes having a tensile modulus of elasticity at least 100 times the shear modulus of elasticity, and assembling a ground contacting tread portion, the reinforced annular band disposed radially inward of said tread portion, at least a first membrane adhered to the radially inward extent of said elastomeric shear layer and at least a second membrane adhered to the radially outward extent of said elastomeric shear layer, and sidewall portions extending radially inward from said tread portion and anchored in bead portions for securing to a wheel.